1. Field of the Invention
The present invention relates to an optical filter element, an optical filter, and a method of manufacturing the optical filter.
2. Description of the Related Art
Presently, color filters based on the three primary colors of red, green, and blue are used to pick up or display full color images in color image pick up devices such as electronic cameras and video cameras or color display devices such as liquid crystal displays and liquid crystal projectors. Specifically, they allow only the colors carried by pixels to pass through corresponding to the respective pixels of image pick-up elements or liquid crystal elements and absorb the other color therein, to thereby pick up the images of the three colors of red, green, and blue. In order to obtain these color filters, pigments having strong absorption power at a specific wavelength are applied. However, the color filters having the pigments have problems in that the selectivity of transmission wavelength is poor, a transmission area is gently expanded due to a wide transmission wavelength area, and it is difficult to arbitrarily set a transmission wavelength area. Furthermore, since such color filters are generally weak against heat and disadvantageously absorb light of wavelengths outside the transmission wavelength area, they are not available under light having high intensity. In order to cope with the problems, there have been proposed a number of methods that use a filter composed of a laminated film, a dielectric multilayer film in which two types of nonmetal transparent materials each having a different refractive index are alternately laminated.
According to the invention as disclosed in Patent Document 1, for example, the dielectric multilayer film can provide various transmission spectral characteristics depending on the design of a film configuration, enhance the selectivity of transmission wavelength, and allow a transmission area to be narrowed.
Furthermore, the dielectric multilayer film has the characteristics of scarcely absorbing light and reflecting light of wavelengths outside a transmission wavelength area. With these characteristics, it can also be used not only as transmission-type but also as reflection-type color filters for liquid crystal display devices as described in the following Patent Documents. The invention as disclosed in Patent Document 2 provides a display device that forms three types of dielectric multilayer films having transmission wavelength corresponding to the three colors of R (red), G (green), and B (blue) for each liquid crystal pixel and produces color displays using the colors of RGB for the transmission-type color filter and those of C (cyan), M (magenta), and Y (yellow) for the reflection-type color filter.
In manufacturing color filters having a different transmission wavelength using the dielectric multilayer film as described above, it is necessary to change the configuration of the laminated film. Although the laminated films are formed by physical vapor deposition (PVD) method, chemical vapor deposition (CVD) method, or the like, they are separately formed to have a different configuration. In order to form color filters each having a different spectral characteristic for each area, it is necessary to bond such separately-formed laminated films together, which in turn causes low productivity and high costs. Particularly, it is very difficult to arrange such different laminated films in a fine area.
As opposed to this, there have been proposed optical filters having different spectral characteristics for each area, which can be manufactured by collective shape patterning. These optical filters eliminate separately forming the laminated films and use either a method of forming the laminated films after patterning shapes on a substrate or that of patterning the shapes on the substrate after forming the laminated films. Since such optical filters need only one step for manufacturing the laminated films, they can be excellent in productivity. According to Patent Document 3, for example, there has been proposed an invention related to a method of using a photonic crystal structure in which a fine periodic structure having different pitches is formed on a substrate and the laminated film is formed thereon. With this invention, it is possible to manufacture optical filters having different transmission wavelength areas by changing the pitch of the fine periodic structure formed on the substrate, namely, the lattice constant of photonic crystals.
Furthermore, Patent Document 4 discloses an optical filter having a structure in which fine cavities are formed in the laminated film. In this optical filter, the pitch of the cavities or the ratio thereof is different for each area, thereby creating a different transmission or reflection spectral characteristic to be expressed for each area. The optical filter of this type can shift a transmission wavelength area to a short wavelength side based on the ratio of the cavities.    Patent Document 1: JP-A-07-270613    Patent Document 2: JP-A-356768    Patent Document 3: JP-A-2004-341506    Patent Document 4: JP-A-2005-26567
As described above, there have been proposed the various optical filters. However, although the optical filter of Patent Document 3, for example, can shift a transmission wavelength area by changing the structure of crystals, it cannot increase the shift amount of the transmission wavelength area. Even optical filters of any transmission wavelength area could not increase the shift amount. For example, it is difficult to shift the transmission wavelength area up to those corresponding to the colors from red to blue only by changing the pitch of the fine periodic structure. Furthermore, the optical filter of Patent Document 4 has a problem in that a stop wavelength area also becomes narrowed as the ratio of the cavities increases. In other words, it is not easy to obtain a sharp transmission wavelength area with the optical filter. Furthermore, the optical filter does not provide sufficient selectivity of wavelength of transmitted light. Moreover, it is difficult to use the optical filter for light having a wide wavelength area such as visible light.
The present inventor has designed the optical filter (6) as shown in FIGS. 1A, 1B, and 2 based on the above inventions and studied its light transmission characteristics. FIGS. 1A and 1B are cross-sectional views schematically showing the structure of the optical filter (6), and FIG. 2 is a table showing the configuration of the laminated film of FIGS. 1A and 1B. The laminated film has a first layer and a second layer in this order from a translucent substrate. In FIG. 1A, a laminated film 0-102 is formed on the translucent substrate 0-101 with periodic gaps (concave parts) cut out in parallel with each other at an interval “p,” and the width of a laminated structure part is represented by a×p. Here, “a” represents a duty ratio that is the ratio of a width in the laminated film structure relative to the interval (also referred to as pitch) “p.” The duty ratio “a” is a value between 0 and 1. The duty ratio of 0 represents no laminated film structure and that of 1 represents a structure where no gaps are formed in the laminated film. The pitch “p” and the duty ratio “a” have the same meanings in the following description as well.
FIG. 3 shows the results obtained by calculating with rigorous coupled wave analysis (RCWA) a spectral transmission factor relative to the structure having the pitch “p” of 0.3 μm as shown in FIGS. 1A and 1B in the laminated film structure as shown in FIG. 2. In FIG. 3, the solid line (a), the dotted line (b), and the chain line (c) show spectral transmission factors where the duty ratio is 1, 0.5, and 0.2, respectively. Note, however, that they show the results of cases where light is perpendicularly incident on a filter and light (hereinafter referred to as TE polarized light) has an electric field vibration parallel with the periodic structure in its polarization direction. In the electric field directions of the TE polarized light and the TM polarized light orthogonal thereto, the TE polarized light is directed parallel with parallel grooves and the TM polarized light is directed perpendicular to the TE polarized light as indicated by the arrows in FIG. 1A.
As shown in FIG. 3, it is possible to change the light transmission characteristics by changing the duty ratio “a” of the periodic structure parts relative to the same laminated film structure. The smaller the duty ratio “a” is, the closer the main transmission wavelength area (hereinafter simply referred to as transmission band in some cases) as indicated by the arrows in FIG. 3 is shifted to the short wavelength side. Therefore, it is possible to easily manufacture an optical filter having different light transmission characteristics just by forming the same laminated film on the translucent substrate and changing the pitch of gaps and the duty ratio “a.” As is clear from FIG. 3, however, if the duty ratio “a” is small, the reflectivity of light at a wavelength around a transmission band may be reduced, a stop wavelength area (hereinafter simply referred to as stop band in some cases) may be narrowed to degrade its stop function, and a light transmission area may even appear particularly on the long wavelength side. The width of the stop wavelength area and the strength of the stop function are based on a difference in refractive index of materials constituting the laminated film. Furthermore, if the laminated film has a fine structure like gaps, its refractive index will approximate an effective refractive index considering the influence by the fine structure. Since the effective refractive index becomes smaller as the duty ratio “a” of the fine structure gets smaller, the difference in the effective refractive index of two types of materials also becomes small in accordance with the duty ratio “a.” Therefore, if the duty ratio “a” is small, the stop wavelength area will be narrowed and the stop function will be degraded.
In other words, the above things imply that it is difficult for such a structure to obtain filters including from one that allows only the color of red to pass through to another that allows only the color of blue to pass through with the same film configuration in attempting to manufacture optical transmission filters corresponding to the wavelength areas of the colors of red, green, and blue. There have not been proposed means for solving the problems. Therefore, with the above method, it is not possible to manufacture optical filter elements used in a wide wavelength area, particularly in the whole visible light area and light transmission filter elements that allow mainly only different wavelength areas such as the colors of red, green, and blue to pass through with location in the laminated film after forming the fine structure therein.